The present invention relates to a sensing device for determining where a sludge layer begins in a settled waste water treatment vessel, so that clarified waste water above the sludge can be removed from the vessel without entrainment of the sludge.
Sequencing Batch Reactors (SBR's) are utilized to treat wastewater, leachates, and other liquid streams from municipal, private, industrial, and combined sources. The design and ultimate cost of SBR's is determined by examining the expected average and maximum waste water flow rates as well as the organic and chemical pollutant content of the wastewater. Typically, one of these factors will determine the minimum size or capacity of the plant and associated equipment. This, in turn, determines the cost of the system and the operational limits as well.
Frequently, the hydraulic load to the plant is highly variable and becomes a limiting factor. This is especially true when plants are designed to handle high storm flows, or when a multiple unit plant is designed properly to continue to operate effectively when one or more of the units are out of service, so that the remaining units must be capable of handling the full flow.
In a sequencing batch reactor process, it is required that clarified fluid be withdrawn after a settling stop from the reactor or digester within which the process is occurring. During certain cycles within the reactor, especially mix cycles, sludge is agitated with all of the fluid in the reactor in order to mix organic and chemical degrading bacteria with materials in the water being treated. It is important that the decanting system not allow the sludge to enter the decanter during the mix cycles and that the sludge not settle within the decanting system to prevent sludge from being withdrawn along with the clarified liquid when the sludge has settled and clarified liquid is withdrawn from the reactor. Sludge entrained with and discharged with the clarified liquid causes substantial downstream pollution and is very undesirable.
One of the major problems with certain prior art decanter systems for use in batch reactors has been that a receiver for the decanter has had the interior thereof open to fluid within the reactor during sludge mixing cycles. When the sludge is being mixed with the incoming effluent and the entire reactor is in a generally mixed state, sludge occurs near the top of the reactor as well as at the bottom. If the receiver is open at this time, the sludge usually enters the receiver and settles therein during a settling cycle. Thereafter, when the clarified fluid is withdrawn through the receiver, the sludge that settled within the receiver is entrained with the clarified fluid to pollute the effluent.
Over the years, engineers have developed various devices to solve this problem. In one such device, an initial quantity of clarified effluent removed from the reactor during each decanting cycle is pumped back into the reactor to thereby return the entrained sludge. However, such a solution requires a pump and control mechanisms which complicate the system and can easily fail, leading to substantial downstream pollution.
Other attempts to solve the problem of sludge settling within the receiver have been directed to physically removing the receiver from the tank during mixing cycles. This typically requires a cumbersome and expensive structure which is suitably strong to hold a decanting system out of the reactor during the mix cycle. In addition, where freezing is likely to occur, fluid from the decanting structure may freeze if raised from the body of liquid in the reactor, or the fluid level at the top of the reactor may freeze which can make it difficult or impossible to raise and lower the decanting structure.
Various types of pressure responsive valves or flaps have been placed in the openings to the decanting system to keep sludge out of the system when the contents of the reactor are being mixed. While such valves or flaps have had varying degrees of success in reducing the sludge that settles in or on the intake of the decanter, such valves and flaps cannot protect against the decanter being lowered into the settled sludge layer itself.
To overcome the problem of lowering the decanter into the sludge layer, some processes simply cease decanting upon reaching a specific pre selected water lever which is determined by a float switch, or by a level sensing device, which is monitored by a computer or microprocessor. In most cases, this type of operation is grossly inefficient. The float switch or level indicator does not provide an indication of the height of the settled sludge blanket within the vessel. The height of the settled sludge blanket does vary with different conditions, especially with respect to how recently sludge was wasted from the vessel. Therefore, if the sludge biomass has not settled below the height where the discharge enters the decanter, highly polluting sludge is discharged to the environment directly. On the other hand, the discharge of clarified supernatant may be stopped at such a selected water level, when in fact a large quantity of dischargeable supernatant may be available below that level.
A reliable system to monitor the sludge level and operate in such a manner that a high percentage of the dischargeable liquid is decanted with every cycle would maximize the treatment capacity of the process, reduce the initial construction cost and size, as well as avoid the inadvertent discharge of sludge.